(1) Field of the Invention
This invention relates to food compositions containing processed green vegetables and, more particularly, to compositions and methods involving processed green vegetable compositions having an improved green color by virtue of the addition of a zinc salt to the composition. The zinc salt is, preferably, a zinc salt of a fatty acid. The processed food compositions and methods therefor are particularly applicable to canned food preparations containing processed green vegetables such as green beans and peas in semi-solid, pureed baby food preparations.
(2) Description of the Related Art
"Freshness" is a universal quality issue with prepared foods, particularly, in the commercial setting. This can be an especially important issue with canned vegetables which must undergo a heat sterilizing process to avoid any hazard to public health whether packed in glass jars, metal cans or some other hermetically sealed container. This is because this process, which preserves the comestible qualities of food and makes seasonal foods available year round, can adversely affect the color of green vegetables. As a result, canned green vegetables do not possess the bright green color of fresh vegetables, but instead have an "overcooked", grayish, olive-drab color. Indeed the greener appearance of frozen vegetables has been touted as being an indication of the freshness of the frozen vegetables compared to canned vegetables and this reflects the desirability for vegetable food preparations to possess a bright green color.
The chemical basis for the loss of the bright green color of fresh vegetables upon thermal processing, has been extensively studied. These studies have shown that chlorophyll, which is responsible for the green color in vegetables, degrades during thermal processing to compounds called pheophytins and pyropheophytins. The formation of pheophytins and pyropheophytins imparts the olive drab color to thermally processed vegetables and the formation of these substances is initiated by the release of cellular acids and the synthesis of new acids during thermal processing. As the mechanism of chlorophyll degradation became elucidated, a number of approaches were suggested for preserving the natural color of processed green vegetables (see LaBorde and von Elbe, J. Agric. Food Chem. 38:484-487, 1990). These approaches have included the addition of alkalizing agents in the process solutions; high temperature, short interval processing; and low temperature blanching. Whereas, these methods have been reported to retain the green color of vegetables immediately after processing, the natural green color is lost during storage.
In contrast to the olive drab color of pheophytins, certain metallo complexes of pheophytins and pyropheophytins have a green color similar to that of chlorophyll. These metallo complexes are formed when a metal such as zinc or copper replaces the magnesium of chlorophyll and upon thermal degradation of chlorophyll, zinc or copper pheophytins and pyropheophytins are formed. This results in a re-greening effect in the thermally processed vegetable in the presence of the metal and such has been observed with a number of different vegetables such as for okra in the presence of zinc ion (Fischbach and Newburger, Assoc. Official Agricultural Chem. 26:134-139, 1943; Fischbach and Newburger, Assoc. Official Agricultural Chem. 26:140-143, 1943), for peas in the presence of zinc or copper ions (Schanderl et al, J. Food Sci 30:312-316, 1965), for brussels sprouts in the presence of copper ions (Swirski et al., J Agr. Food Chem. 17:799-801, 1969) and for green beans in the presence of zinc or copper (Decleire, Rev. Ferment. Ind. Aliment. 21:95-98, 1966). The use of zinc or copper salts has been proposed as a possible method for improving the green color of canned green vegetables. (see for example U.S. Pat. No. 4,473,591 to Segner et al, U.S. Pat. No. 5,114,725 to Leake et al. and U.S. Patent No. 5,482,727 to LaBorde et al.). Nevertheless, these earlier reports on the re-greening effect of zinc or copper ion achieved this effect by addition of high levels of water-soluble salts of zinc or copper such as zinc chloride or copper chloride. However, the requirement for high levels of zinc or copper has prevented the application of the re-greening effect in most commercially processed green vegetable foods. This is because the FDA does not currently permit the addition of high enough amounts of these cations to vegetables when added in the form of a water-soluble salt. In particular, the addition of zinc ions is permitted only up to a level of 75 ppm, however, levels of 75 P.P.M. or less have not generally been very successful in producing a re-greening effect. Nevertheless, several reports have indicated that in some preparations, addition of zinc to achieve concentrations below 75 ppm has produced some re-greening effect. For example, Segner et al. (U.S. Pat. No. 4,473,591) reported on incorporation of levels as low as 25 ppm into beans using 190 ppm in the blanching solution, Leake et al. (U.S. Pat. No. 5,114,725) taught that zinc should be incorporated to achieve a concentration of at least 40 ppm and up to 75 ppm although no data on any re-greening effect at these levels was reported and LaBorde et al. (U.S. Pat. No. 5,482,727) reported an improvement in green color at a level of added zinc as low as 51 ppm although this patent indicates that the zinc concentrations in the optimally-colored samples exceeded the FDA limit of 75 ppm.
LaBorde and von Elbe (LaBorde and von Elbe, J. Agric. Food Chem 42:1096-1099, 1994) recently reported on an additional action which was predicted to enhance the zinc complex formation and re-greening effect in processed green vegetables. This group reported that the addition of sodium dodecyl sulfate, sodium caprylate or sodium oleate increased the amount of zinc pheophytin and zinc pyropheophytin complex formation produced upon chlorophyll in the presence of zinc ion. This enhancing effect was shown to occur in a pea puree preparation to which the water-soluble zinc salt, zinc chloride, was added to produce a zinc ion concentration of 300 ppm. The authors of this paper attributed the enhancing effect to a surface-active action of sodium dodecyl sulfate, sodium caprylate and sodium oleate and suggested that surface active compounds may be useful in increasing zinc complex formation in zinc-processed green vegetables.
In spite of the numerous approaches to providing an improved green color to canned vegetables, commercial production of canned green vegetables using added zinc salts to produce a greener color has not been successful because the amount of zinc required to yield a satisfactory color after processing has resulted in zinc concentrations above the FDA limit of 75 ppm (id.). Thus, there remains a continuing need to provide an effective approach for improving the green color of processed green vegetables.